The present invention relates to load-accelerating drives incorporating variable-filling fluid couplings, and more particularly to variable-filling fluid coupling drive in which the accelerating torque imposed n a high inertia load at any instant during the acceleration from rest up to working speed does not exceed a predetermined value. One field in which such a requirement arises is to be found in drives for long conveyor belts. Such conveyor belts, which may be of several miles in length, are used for example for conveying minerals from a mine to a railhead or harbour. Considerable economies can be made in the capital cost of the conveyor belt by reducing the number of belt plies so that the belt will for example withstand forces up to but no more than say 50% greater than the normal operating values. To prevent damage to the belt, the belt drive must be prevented from exerting forces greater than 50% of the normal full load value. Another application where the same requirements apply would be a large fan where the application of excessive driving torques could cause damage to the fan. Furthermore, in the case of electric motor drives, limitation of the maximum torque applied to the load and thus of the maximum torque applied to the motor can prevent undue disturbance of the electrical network and can also prevent excessive voltage drop where the electric motor is situated in a remote location requiring long power lines.